Amplification of electrical currents



May 19, 931. F. s. M CULLOUGH AMPLIFICATION OF ELECTRICAL CURRENTS Filed Nov. 2, 1926 WNW numm

m INVENTOR M, fiiiflw Patented May .19, 1931 FREDERICK S. MGCULLOUG-H, OF EDGEVJOOD, PENNSYLVANIA AMPLIFICATION OF ELECTRICAL CURRENTS Application filed. November 2, 1926.

This invention relates to the amplification of electrical currents, and particularly to systems of radio frequency amplification.

In the amplification of radio. frequency currents such, for instance, as the amplification of received radio signals prior to rectification in the detector, it is a difiicult matter to keep the system free of self-sustained or regenerative oscillations. In order to overcome these oscillations, various methods of neutralization have been resorted to, including circuits for neutralizing the internal capacity of the thermionic tubes'and for effecting a reverse phase feed-back.

While it is necessary, in order to obtain good reception, to prevent such inter-stage oscillations, the usual methods of neutralization are not satisfactory for the reason that they reduce the eiiiciency of the amplifying tubes and reduce the range and selectivity of the apparatus in which they are used.

Another disadvantage arising from most methods of neutralizing the tubes requires that the tubes be perfectly matched, both as to their mutual conductance and amplification characteristics, as well as to their internal capacities.

According to the present invention it is proposed to overcome the difficulties encountered in systems for effecting radio frequency amplification, which is more simple than the present methods of neutralizing, and which, at the same time. increases the efficiency and selectivity of the circuit in which it is used. v

According to the present invention it is proposed to overcome the tendency toward inter-stage oscillations by means of tubes having different internal capacities, the internal capacities of the tubes decreasing toward the output end of the system.

The invention may be readily understood by reference to the accompanying drawing, which illustrates diagrammatically one circuit employing the principles of my invention.

In the drawing I have shown a common type of circuit employing a plurality of K0 stages of tuned radio frequency amplification. In the draw1ng,'2 designates the pri- Serial No. 145,775.

mary of the first radio frequency transform- I er whichis connected across the aerial and ground. The secondary 3 of this transformer has one end connected to the grid 4 of the first amplifying tube le the other end of the secondary is connected to a common filament wire ,6 into which one side of the cathode 7 of the tube 5 connects; The tube 5 has a plate .8 which connects with one end H of the primary 9 of the next radio frequency transformer in the usual manner. The secondary 10 of the second radio frequency transformer is connected in the usual way to the grid 11 ofthe second thermionic tube The plate 14 is shown as leading to the pri- 12having'filament or cathode 18 and a plate mary 15 of another radio frequency trans- I former having a secondary 16. 1 One end of I the secondary 16 is connected to the grid 17 of-a third tube 18 having a filament 19 and.

a plate 20. The outputof the tube 18 may lead to succeeding stages of radio frequency amplification or to a detector tube, or to any other preferred circuit.

The circuit itself is of the type commonly employed for effecting tuned radio frequency amplification, each of the stages of radio frequency amplification being provided with a tuning condenser 21. The values of'the inductances and capacities are uniform for each stage of radio frequency "amplification, that is to'say, for instance, the primaries 2, 9 and 15 of the respective radio frequency transformers are of corresponding inductance, as are also the secondariesS, 10 and 16.

According to the present invention, the tube 5 has 'a greater internal capacity than the tube 12, and thetube 12 in turn has a greater internal capacity than the tube 18. For 'instance, the first tube 5 may have the same internal capacity as the usual tube now employed in circuits of this kind. The second tube 12 has a slightly smaller capacity between its elements, while the third tube 18 has a smaller capacity b-etween'its elements. This has been illustrated diagrammatically in'the "drawing by the variations in the sizes of the grids and plates of the respective tubes.

still capacity between the grid and the plate thereof. The grid element and the plate element being arranged parallel to each other and in close proximity, form a capacity coupling between the input circuit of the tube and the output circuit of the tube. It is by reason of this internal capacity that V thereis, a tendency for the stages of radio frequency amplification to oscillate, and it is because of this internal capacitythat it is difiicult to prevent oscillations in systems of radio frequency amplification. By reason of this capacity coupling between'the grid and the plate, radio frequency currents flow between the grid and the plate directly from one tube to the next Without being amplified and without responding to the tuning elements in the circuit. t

According to the present invention, the first tube passes some radio frequency energy through it which is not amplified, due to the relatively large; internal capacity of the tube, but since the internal capacity of the second tube 12 is smaller than that of the first tube,

the unamplified energy from the first tube cannot all pass through the second tube. The second tube, therefore, will not pass as much unamplified energy to the third tube as the first tube passes to, the second. Therefore, the second tube will not only amplify the output or amplified signal of the first tube, but'it will also amplify some of thetion having a plurality of amplifying stages, 7

each of said stages having a thermionic tube,

the capacity coupling between the grid and the plate in each succeeding stage being decreased toward the output end of thecircuit.

3. A system of'radio' frequency amplification having-a plurality of successive amplifying stages, each stage including a thermionic tube, the capacity coupling between the grid and plate in the first and last stages being difierent and being less intthe last stage than in the first. i s i In testimony whereof I have hereunto set my hand. FREDERICK S. McCULLOUGH.

energy thatis bypassed in thefirsttube by V reason of the capacity coupling between the grid and the plate. In the same way the third tube will amplify the output of the second tube, and in addition, amplify some of the bypassed energy of the second tube. By this system it can be seen that the further this is carried out, and if the'tubes have the same mutual conductance and amplification constant, that excellent amplification can be secured. By reason of the gradient in the internal capacities of the successive tubes, the tendency toward self-sustained interstage oscillations is. prevented, while the selectivity of the circuit is at the same time increased. It will therefore be seen that according to the present invention there is provided a system of radio frequency amplification which does not require neutralizing, and 

